The rotary drilling of a deep oil or gas well requires that many joints of drill pipe having enlarged screwed end connections, be raised, lowered and rotated for months so as to drive the drill bit against the bottom of the hole. During such a process, the wellhead assembly bore near the top of the casing string being drilled through, may be severely damaged by members of the drill string such as drill pipe, tool joints, bits and drill collars as the drill string is lowered or rotated within the wellhead bore. Maximum damage to the wellhead assembly bore usually occurs in a zone immediately below the level that the wellhead assembly bore reduces to the bore of the casing string being drilled through, the bore of the casing head, the blow out preventers, the drilling spool and such that may be a part of the wellhead assembly, being greater. Such damage results in loss of metal from the casing wall to thereby weaken the casing and vastly increase the danger of blowouts and even loss of the well.
So as to prevent or at least reduce such damage to the wellhead assembly bore, various devices have been used above the level that the wellhead assembly bore substantially equals the casing bore, to guide the drill string members from the larger upper bores of the wellhead assembly and into the casing bore below, such as: retractable guide ram members mounted in a drilling spool; a ring member having a bore therethrough substantially equal to the casing bore, having an outer diameter sufficient to pass through the wellhead assembly above the level that the wellhead assembly bore substantially equals the casing bore, the ring member having guide surfaces from the outer diameter to the bore therethrough; a removeable ring member as described but having an outer diameter greater than the upper wellhead assembly bore and mounted above the wellhead assembly. Devices as described above have served well under certain conditions, however each possess undesirable characteristics that prevent their successful use, such as: when the casing bore is substantially less than the bore of the upper portion of the wellhead assembly, projection of guide rams may be excessive and thereby vulnerable to damage from drill string members; when the wellhead assembly is forced from a normal vertical mounting to a leaning position by drilling forces and by vibration, the ring member mounted above the wellhead assembly can no longer serve as a guide into the casing bore, the drill string hanging vertically; the ring member that is lowered into the wellhead assembly being subject to vibration and impact from the drill string such that the ring member is displaced from its normal guide position so as to prevent proper guiding of the drill string members. Therefore, features of the present invention are to: provide protection for the bore of a wellhead assembly against damage caused by drill string members; provide for protection for the upper zone of a casing string against damage caused by drill string members; provide means for removably mounting an apparatus for protecting the bore of a wellhead assembly against damage caused by drill string members such that the apparatus will be held securely in position during drilling operations; provide for extended protection of the casing bore by means of a portion of an apparatus positioned within an upper section of the casing bore; provide for means for automatically centering the apparatus with respect to the casing bore.